It is frequently desirable to remove particulate matter from flowing streams to reduce contamination of downstream processes and equipment. In the gas-shielded arc welding field, welding spatter is a particularly acute problem because of the regular spherical shapes and the small size, typically 10 .mu.m diameter, of the particles. These particles must be removed from the gas flowing through a regulating valve because they may deposit on valve seats and reduce the accuracy and reliability of the regulator mechanism. Several types of filters have been used in gas regulators. Among these filters are wire mesh screens which must be made of very small diameter wire since spherical particles having a diameter less than about one-quarter of the diameter of the wire can pass through a closely woven wire mesh. Smaller particles can be trapped if the woven wire is mechanically flattened or compacted to decrease the space between the wires. Unfortunately, such compacting also reduces the area open for gas flow and increases the pressure drop across the filter. Further, trapped particles tend to build up at the sharply defined surface of the screen, and the resulting clogging further increases the pressure drop required to maintain a desired gas flow rate. Natural and synthetic fiber fabrics have also been used as filters. Further, clumps of natural, synthetic, or metal fibers, e.g., steel wool, have been used as filters in gas regulators. These types of filters have randomly shaped openings between the fibers and provide relatively poor filtration for small particles. The performance of a sintered metal filter suffers from the same problem for a similar reason.
Fiber glass has also been used as a material for regulator filters. This material has special properties which make it a preferred material for such applications. More particularly fiber glass is non-flammable, non-hydroscopic, chemically inert, non-toxic, durable, inexpensive, readily available, and very effective. Two types of fiber glass filters are known: (1) A clump of short fibers formed into a wad and stuffed into a cavity. Typically, the length and diameter of the loose roving was 50 to 100% greater than the diameter of the cavity; (2) A disc cut from a fiber glass blanket in which the fibers are randomly oriented in layers with a thin layer of sizing applied to the surface layers to assist in retaining disc shape during cutting and handling. The diameter of the cut disc was approximately the diameter of the filter cavity.
Both of these previous fiber glass filters suffered from the disadvantage that the filter comprised relatively short lengths of glass fiber and thus included a very large number of cut ends. Such ends are easily broken and create a nuisance and possible hazard during fabrication. In addition, experience with the blanket type filters revealed that they would greatly compress and expand as the flow rate of the gas increased and decreased. For example, tests with a fiber glass blanket filter having an initial thickness of 0.5 inches revealed that it compressed to approximately 0.035 inches thickness under typical flow conditions, and then expanded back to about 0.25 inches when the flow stopped. The resulting mechanical movement breaks the cut ends of the glass fibers and the pieces become contaminants to be trapped. In extreme situations the movements can cause the filter to dislodge the metal screens or other structure which supports the filter in the fluid line.